A metal blank that is put temporarily inside a bushing and screwed onto the end of a conduit run. By doing this, dirt, plaster, nails, etc. are kept out of the conduit system during construction. When connecting to complete the installation, the penny comes out and you re-use the bushing. Today, it is made in metal but it used to be available in paper and wood. Also known as "push penny" and was usually made out of thin cardboard.